1. Field of Invention
This invention relates to laser modulators, and more particularly to broadband microwave waveguide modulators for infrared (IR) lasers.
2. Description of the Prior Art
The use of a combination bulk electrooptic crystal and a microwave waveguide to provide electrooptic modulation of an infrared (IR) laser is known in the art. The electrooptic crystal functions as an optical waveguide and the modulation of the IR beam is produced by a modulation signal subcarrier at microwave frequencies which modulate the index of refraction of the optical waveguide medium through a nonlinear electrooptic effect in the waveguide that results in a phase shift modulation of the IR beam. The phase shift modulation creates a power conversion of a portion of the IR carrier signal into optical sideband signals. The modulated IR beam (f.sub.o) is comprised primarily of the input IR carrier frequency (f.sub.c) and the first order upper and lower sideband frequencies produced by the modulation signal (f.sub.m), i.e. f.sub.o =f.sub.c .+-.f.sub.m, each occurring within a modulation bandwidth (B) defined as the frequency range of the optical sideband signals between the -3 db, or half power points. Due to the thickness of the bulk crystal, the conventional bulk electrooptic crystal modulator requires extremely high modulation input power levels to provide the required electric field intensity within the crystal resulting in a low conversion efficiency, i.e. the ratio of laser side band power to the modulation signal driving power
Subsequent development of planar waveguide modulators using thin-film electrooptic crystals provided for improved conversion efficiency of the modulator. With decreasing film thickness, the modulation signal input power in the thin-film waveguide modulators can be reduced in proportion to the thickness while still maintaining the same modulation power density. The typical driving power required to obtain the required modulation field intensity in the thin-film crystal waveguide is on the order of one hundred times less than the driving power required in the bulk crystal devices. In addition, the thin-film waveguide alleviates the problem of beam diffraction and alignment encountered with bulk crystal waveguides so that the length of the interaction region (that portion of the optical propagation path in the optical waveguide which is common with that occupied by the electric field intensity produced in the waveguide by the modulation signal) can be significantly increased.
A standing wave, or resonant microwave modulator incorporating a thin-film electrooptic waveguide in a microwave modulator structure was first reported in an article entitled Microwave Modulation of CO.sub.2 Lasers and GaAs Optical Waveguides, by P. K. Cheo and M. Gilden, Applied Physics Letters, Volume 25 No. 5, Sept. 1, 1974, Pages 272-274. There the generation of a CO.sub.2 laser sideband power at frequencies .+-.10 GHz from the laser carrier frequency was provided through use of a minigap microwave ridge waveguide modulator loaded with a thin-film Gallium Arsenide (GaAs) slab between the minigap ridge and a base plate. The microwave ridge modulator increased the power conversion efficiency by a factor of 300 over that obtained with a bulk crystal modulator and the length and width of the interaction region was on the order of 1 cm and 25 microns respectively, which allowed a high microwave field intensity to be sustained at either X band or Ku band within the entire interaction region. Similarly, a traveling wave modulator structure using a thin-film electrooptic waveguide in a traveling wave minigap microwave ridge structure was reported in an article Thin Film Waveguide Devices, by P. K. Cheo, Applied Physics, Vol. 6, pages 1-19 (1975). The distinguishing performance characteristics between the resonant standing wave modulator and the traveling modulator, centers on the power conversion efficiency and modulation bandwidth. The standing wave structure provides higher power conversion efficiency, while the traveling wave device provides a larger modulation bandwidth.
The disadvantages on both standing wave and traveling wave microwave ridge modulators is the inability to provide a high degree of electrode to waveguide surface contact, which results from surface imperfections in the waveguide material. The lack of intimate electrical contact between the electrode surface of the ridge and the surface of the waveguide creates air gaps between the surfaces which result in impedance mismatch between the output impedance of the microwave source and the effective input impedance of the microwave ridge waveguide. In addition to the impedance mismatch, the airgap causes an increase in the velocity of propagation of the microwave signal in the interaction region since the airgap and the waveguide create a composite medium with an index of refraction less than that of the waveguide itself. This results in nonsynchronization of the modulating signal with the optical wave and a direct degradation in modulator performance.
The problems of large physical size and inefficiency present in the mini-gap waveguide modulator are eliminated in an integrated modulator structure in which the microwave electrodes are electroplated directly on the top and bottom surfaces of the thin-film optical waveguide. Such integrated structures have been developed using thin-film planar optical waveguides, and provide a far greater conversion efficiency than that of the bulk crystal devices or the mini-gap waveguide modulator structure. These planar waveguide devices, however, suffer from degradation in the optical coupling efficiency and the inability to provide exact confinement of the optical wave through the modulation field. This results in optical distortion in the modulated wave and less than the fully realizable conversion efficiency levels.